The phrase “single-phase fluids” as used herein means either liquids with dissolved gases, or mixtures of liquids having different vapour pressures. Such liquids can be separated into their component parts either by taking the dissolved gas out of solution or, in the case of mixtures of liquids having different vapour pressures, by converting one of the liquids to vapour form and then separating it from the remaining liquid. The original single phase fluid can thus be converted into separate gas and liquid phases. It should be noted that while the term “single phase fluid” refers essentially to liquids of the types described above, it is not intended to exclude fluids that include such liquids in combination with some free gas, for example in the form of bubbles. In this latter case, the invention may serve to separate the free gas from the liquid while simultaneously separating the gas portion from the liquid portion of the single phase fluid.
Dissolved gases are frequently present in liquids in their natural form. For example, raw crude oil usually contains some dissolved hydrocarbon gas. Air or other gases may also become dissolved in liquids during their production, processing or transportation. For example, chlorine gas may be added to water during treatment. It may be necessary to remove some or all of this dissolved gas prior to processing, transportation or storage. For example, in the case of oil, if the dissolved gas is not removed, it may subsequently be released by agitation during transportation or by a reduction in pressure, leading to a potentially dangerous build-up of explosive gas in containers, tankers or other sources handling such fluids.
One widely-used method of degassing liquids is to pass the liquid through a separator vessel in which the pressure of the fluid is reduced to below atmospheric pressure. As the pressure is reduced the dissolved gas comes out of solution and rises to the surface of the liquid as bubbles. The evolved gas can then be removed and separated from the remaining liquid. This method is used in the oil and gas industry to remove dissolved hydrocarbon gases from liquid crude oil before it is sent to storage tanks or to tankers for export.
The system described above is however complex and bulky, requiring large separator tanks and vacuum pumps or multi-stage eductors (i.e. ejectors or jet pumps) and compressors to generate the required low pressure. A pumping system is then needed to boost the pressure of the degassed liquid back to the level required for transportation by pipeline to a storage tank or tanker. The pressure of the separated gas phase, which is at or below atmospheric pressure, may also have to be boosted using a compressor or eductor/jet pump, so that it can be transported or flared.
A similar method may also be used for separating mixtures of liquids having different vapour pressures. Lowering the pressure of the mixture to below the vapour pressure of one of the liquids causes that liquid to be transformed into a free gaseous phase, which can then be separated from the remaining liquid. This method is commonly used for removing chemicals from mixtures of liquids.
A cyclonic separator is described in international patent application No. WO99/22873A. The device is designed primarily for separating dust particles from air in a vacuum cleaner, although it may also be used for separating mixtures of gases and liquids. During use, a vortex is created having a radial pressure gradient with a low pressure at the centre of the vortex and higher pressures at greater radii. A reduction in pressure can thus be achieved along the axis of the separator within its central core.
There is no suggestion that the above said cyclonic device can be used for degassing liquids. However, even if the separator could be driven hard enough by increasing the flow rate through it to cause some dissolved gas in the liquid to come out of solution, the separator is not designed for this use and the maximum reduction in pressure that can be achieved (to approximately 0.9 bar) is not sufficient for efficient separation of dissolved gases. The separator is also only able to operate over a relatively narrow range of flow rates.